Broken Dorsal

Arrgh!  I woke up this morning and found out that my newest Ryukin broke the tip of his dorsal fin.  I have no clue how he did it.  There's only two filters and a "sponge filter" powered by an air tube [like this] (they're more like BB collectors than anything.)  At first I was annoyed because, well, it's like chipping a $200 vase, for the lack of a proper and less materialistic comparison.  However, it probably hurts him like the dickens and it's really not his fault. :(  I've added .01% salt and am watching the water. He'll be okay, though.